Many types of speaker diaphragms are well-known in the prior art and are used to convert electric vibration into mechanical vibration. The most common type is the cone diaphragm which directs acoustic sounds by mechanical vibration of the cone, Nilson, U.S. Pat. No. 1,748,990, Bostwich, U.S. Pat. No. 1,917,013 and LaRue, U.S. Pat. No. 1,798,688.
Pak, U.S. Pat. No. 5,256,837 discloses a speaker cone formed by the steps of cutting a sheet of paper into a pattern simultaneously with seaming several folding lines into wing portions to reinforce the cone, adhesive bonding the facing surfaces of the folded portions, and, coating the entire surface of the article with moisture proof material such as tar, oiled paint, or enamel.
Rehde et al, U.S. Pat. No. 4,100,992 provides a loudspeaker including a porous conical diaphragm fixed in its central zone to the bobbin of an electromagnetic drive element. The diaphragm is totally permeated with a varnish of cellulose type to serve as a stiffening material. This varnish penetrant is disclosed to be of sufficient fluidity to permit penetration into the texture of the very thin membrane forming the diaphragm without increasing the thickness. The permeated portion of the diaphragm may also be limited to a circular zone around the ring. The diaphragm may carry, outside the permeated zone, various further zones covered by hard or stiff material, with these latter zones being independent of each other and extending out to the outer edge of the diaphragm. Thus the outer zone of the diaphragm can be provided with a series of three to eight radial bands or strips such that the loudspeaker diaphragm has substantial rigidity or stiffness during the propagation of mechanical vibrations.
Garner et al, U.S. Pat. No. 4,020,299 discloses the general form of a cone for a conical diaphragm loudspeaker. The narrow end of the cone forms a small aperture to which a voice coil is attached. The voice coil is carried in the magnetic field of a permanent magnet and causes oscillation of the diaphragm when a suitable exciting current signal is fed thereto. The conical diaphragm is provided with four substantially radial bands of perforations filled with a damping material said to have having a high level of internal energy absorption to oppose the formation of standing waves in the diaphragm. The composition of the dampening material is unspecified except that it is ambiguously described as a p.v.a. based compound.
In the prior art, paper is commonly used as the diaphragm material which is accomplished by forming the cellulose fibers of paper to a certain configuration using a mold. Alternatively a thermoplastic material may be used. One such material previously suggested for this purpose is a rubber modified styrene which can be vacuum formed into a three dimensional shape. The thickness of such a diaphragm would be in the region of 0.2-0.4 mm.
The diaphragm should have low frequency response characteristics and high flexural wave propagation velocity and retain these characteristics over the life of the cone. Environmental factors such as heat and humidity appear to cause a loss of the desired acoustical characteristics over time.
The present invention relates to the treatment of the speaker diaphragm, preferably composed of paper or plastic and having the typical conical configuration, after it has been formed to extend its original performance life by the application thereto, as a layer or film, or by impregnation, of a polymeric material as more fully described hereinbelow.